The invention relates to imaging a curved object in a panoramic array of data, and especially, but not exclusively, to panoramic imaging of the dental arch. The invention has particular application to imaging the maxilla, the mandible, or both in a dataset of part of the head of a human or other mammal.
In certain forms of dental medicine and surgery, a “panoramic” image of the jaw is used to examine the jaw, for example, for monitoring of dental health and condition, diagnosis, and planning of prosthetic and other surgical procedures. The panoramic image of the jaw, like a panoramic photograph, depicts the jaw as if it were imaged onto an imaginary approximately cylindrical sheet with the axis of the sheet upright, and the sheet were then unrolled into a flat form. However, the human jaw is not a perfect circular arc, so the “cylindrical” shape of the imaginary sheet is not exactly circular.
A panoramic x-ray image is conventionally made by placing the target between a source of x-rays and a detector of the x-rays and causing the source and the detector to rotate around the target. The source is collimated to produce a narrow fan of x-rays in a plane parallel to the axis of rotation, and the detector is arranged to expose a narrow strip of detecting surface aligned along the fan of x-rays. X-ray absorbing structures in the target cast an x-ray shadow on the detector strip. Any structure that is not exactly on the axis of rotation casts a shadow that moves across the strip as the source and detector rotate. In a conventional panoramic x-ray machine, the detector strip is part of an x-ray sensitive photographic film, exposed through a slit in a metal mask. By moving the film across the slit at a controlled speed, a part of the structure at a specific distance from the axis of rotation can be brought into sharp focus, because the shadow of the selected part of the structure follows the movement of the film exactly. More recent panoramic x-ray machines use a charge-coupled device (CCD) or a CMOS detector array, and a similar panoramic image can also be produced utilizing an Amorphous Silicon Flat Panel detector, which is also used in Cone Beam CT systems. By stepping the acquired charge across the detector array and integrating the image at a controlled speed, the focusing action of a moving film can be exactly imitated.
The human jaw is not an exact arc of a circle, so a simple circular motion of the source and detector, with the source, detector, and film transport or CCD stepping moving at a constant speed, does not provide a well-focused image of the whole jaw. Panoramic x-ray devices have been developed in which the focus position is adjusted so as to follow the actual curve of the jaw, either by a complicated movement of the x-ray source and/or detector, or by varying the rate of CCD readout, or both. However, such devices can adjust the focus position only at the time of x-ray exposure, and only to a pre-programmed jaw contour. If the actual jaw does not match the pre-programmed contour, or if the jaw is not in exactly the expected position relative to the scanner, an incorrectly focused image may result, in which the plane of sharp focus diverges from the jaw for all or part of the dental arch.
The depth of focus can be adjusted by adjusting the width of the detecting strip. A narrower detector strip gives a deeper area of sharp focus, at the expense of requiring either a more intense or a slower exposure to achieve the same level of saturation of the x-ray film or CCD pixels.